If you didn't soak a garment in soda ash before spraying on procion mx, can you do something after the fact to fix the dye?

Yes, apply soda ash now. Spray it on, or paint it on, or dip the fabric into a bucket or tray of soda ash solution to which a large amount of ordinary table salt has been added (to reduce the solubility of the dye that has dried on the fabric, so that less of it washes away into the soda ash).

Soda ash, dissolved in water, can be applied before the dye, mixed in with the dye, or applied afterwards. The reaction between the dye and the cotton will occur only after the soda ash (or another high-pH chemical) is added.

Another option, for hand-painted fabrics, is to buy some liquid sodium silicate and paint it on. It fixes the dye in just the same way as soda ash. Sodium silicate is also known as water glass; Dharma sells it as "AfterFix", and ProChem as "PRO Fix LHF". (This product is not related to Jacquard's Permanent Dyeset Concentrate, which is not suitable for use on cotton or with Procion MX dye.)